1. Field of the Invention
The present invention relates generally to a reflection-type variable optical attenuator, and more particularly, to a variable optical attenuator having a birefringent element for separation/synthesis, a lens and a reflecting mirror, with an input port and an output port which are disposed toward the birefringent element for separation/synthesis, the variable optical attenuator capable of adjusting the amount of attenuation of incident light through control of the rotational angle of the polarization direction by use of a variable polarization rotating device interposed between the birefringent element for separation/synthesis and the reflecting mirror.
2. Description of the Related Arts
Optical communications systems or optical measurement systems necessitate optical attenuators for controlling the amount of transmitted light. This type of optical attenuators typically employ an opposed configuration having a polarizer and an analyzer disposed anterior (input side) and posterior (output side), respectively, to the optical axis of a Faraday rotational angle varying unit. The Faraday rotational angle varying unit incorporated therein serves to apply an external magnetic field to a Faraday element (e.g., a magnetic garnet single crystal film having Faraday effect) by use of, e.g., an electromagnet and vary the external magnetic field applied, thereby providing a control of the Faraday rotational angle of light beams passing through the Faraday element. The optical attenuators variably control the amount of attenuation of light through the control of the Faraday rotational angle.
Such optical attenuators could use, in principle, composite polarizing prisms as the polarizer and the analyzer, although use of the composite polarizing prisms would substantially halve the amount of incident light due to the presence of the polarizer. Thus, it would normally be practical for fiber coupled devices to employ polarization non-dependence type using a pair of wedge-shaped birefringent crystal plates (e.g., rutile crystals).
Incident light from an input fiber passes through the polarizer, the Faraday element of the Faraday rotational angle varying unit, and the analyzer in the mentioned order, and is coupled to an output fiber. The Faraday rotational angle is varied through the control of the variable magnetic field applied to the Faraday element, to thereby control the amount of light passing through the analyzer.
In the optical attenuator of the structure using the wedge-shaped birefringent crystal plates in a pair as the polarizer and the analyzer, however, the dimensional accuracy of the paired wedge-shaped birefringent crystal plates is essential, and poor pair accuracy adversely affects isolation characteristics. For this reason, the machining is extremely difficult, requiring a lot of cost of time and labor. Furthermore, the assembling step places importance on the accuracy of positioning in the direction orthogonal to the optical axis of the wedge-shaped birefringent crystal plate, and possible positional offsets may also adversely affect the isolation characteristics. Thus, the assembly adjustment also needs a lot of cost.
Of late years, the wavelength division multiplexing communications are being put to practical use, as a result of which the optical attenuator has come to be incorporated for each wavelength to equalize the insertion loss. In such a use, it is important in particular to achieve the manufacture at low cost as well as reduction in dimensions. Use of the wedge-shaped birefringent crystal plates is disadvantageous in this respect.
Due to such a conventional optical attenuator's opposed configuration allowing light to travel from one end of the device to the other (i.e., configuration where the input fiber and the output fiber are positioned at opposite ends), a wider space is needed to accommodate fibers excepting the case where the input and output are oppositely arranged, resulting in a reduced degree of freedom of mounting. Furthermore, for the electromagnet, a constituent element of the Faraday rotational angle varying unit, a large installation space is required in the direction orthogonal to the optical axis, making it hard to reduce the thickness of the unit.